1. Field of the Invention
The present invention relates to a dielectric resonator apparatus, and in particular, to a dielectric resonator apparatus comprising at least one dielectric resonator coaxial resonator and having connection conductors for external couplings, wherein the dielectric resonator apparatus is preferably used as a dielectric filter.
2. Description of the Prior Art
FIG. 4A is a perspective view of an appearance of a conventional comb-line type dielectric resonator apparatus having a rectangular-parallelepiped-shaped dielectric block 1, FIG. 4B is a cross-sectional view along a line IVB--IVB' of FIG. 4A, and FIG. 4C is a cross-sectional view along a line IVC--IVC' of FIG. 4A.
Referring to FIGS. 4A, 4B and 4C, three circular cylindrical resonator holes 2a, 2b and 2c are arranged in parallel to each other in the dielectric block 1 so as to penetrate the dielectric block 1 between a pair of first end second end surfaces which are disposed oppose to each other, so that each of the resonator holes 2a to 2c has an opening on the first end surface 1a, and has another opening on the second end surface 1b as seen in FIG. 4C. Further, an outer electric conductor 5 is formed on all the surfaces of the dielectric block 1 as seen in FIG. 4A. In the specification, an electric conductor is referred to as a conductor hereinafter. Inner conductors 3a, 3b and 3c shown in FIGS. 4A and 4B are formed respectively on the inner peripheral surfaces of the resonator holes 2a, 2b and 2c, so that a conductor-non-formed portion or gap 30 shown in FIG. 4C, where each of the inner conductors 3a to 3c is not formed, is formed in the vicinity of the first end surface 1a and also another end of each of the inner conductors 3a to 3c located on the side of the second end surface 1b is electrically connected to the outer conductor 5, as shown in FIG. 4C. The longitudinal length of each of the inner conductors 3a to 3c is set to a quarter of the guide-wavelength .lambda.g/4. Furthermore, a pair of input and output electrodes 14a and 14b shown in FIG. 4A is formed respectively on a predetermined side surface 1c of the dielectric block 1 so as to be electrically insulated from the outer conductor 5 and so as to be close to one ends of the inner conductors 3a and 3c located in the side of the second end surface 1b of the dielectric block 1.
In the conventional dielectric resonator apparatus having such a structure, three quarter-wavelength dielectric coaxial resonators are constituted corresponding to the inner conductors 3a to 3c, and as shown in FIGS. 4B and 4C, an external coupling capacitance Ce shown in FIG. 4B and 4C is formed between the input electrode 14a and the inner conductor 3a, and another external coupling capacitance Ce shown in FIG. 4B is formed between the output electrode 14b and the inner conductor 3c. In the conventional dielectric resonator apparatus, the above-mentioned external coupling capacitances Ce can be changed by adjusting the areas of a pair of input and output electrodes 14a and 14b, or by adjusting the distance between the input electrode 14a and the inner conductor 3a and the distance between the output electrode 14b and the inner conductor 3b.
In the case where the above-mentioned external coupling capacitances Ce are changed by adjusting the areas of a pair of input and output electrodes 14a and 14b, when the external coupling capacitances Ce are increased by increasing the areas of a pair of input and output electrodes 14a and 14b, this results in a decrease in the unloaded Q (Q.sub.0) of the conventional dielectric resonator apparatus.
On the other hand, in the case where the above-mentioned external coupling capacitances Ce are changed by adjusting the distance between the input electrode 14a and the inner conductor 3a and the distance between the output electrode 14b and the inner conductor 3b, it is necessary change the sizes of the dielectric resonator apparatus such as the sizes of the dielectric block 1, the sizes of the resonator holes 2a to 2c or the like.
Accordingly, it is difficult to obtain a predetermined desirable optimum unloaded Q (Q.sub.0) in the electric characteristics of the conventional dielectric resonator apparatus. On the other hand, in order to manufacture various kinds of dielectric resonator apparatuses having various kinds of electric characteristics, it is necessary to manufacture many dielectric resonator apparatuses having different sizes corresponding to the respective electric characteristics. This results in increase in the manufacturing cost and the managing cost.
Further, when the external coupling capacitances Ce are increased by increasing the inner diameters of the resonator holes 2a to 2c, the mechanical strength of the dielectric block 1 decreases.